1. Field of the Invention
The present invention relates to a flat solid oxide fuel cell stack and, more particularly, to a solid oxide fuel cell stack with a uniform flow distribution structure and a metal sealing member, in which fuel and air introduced into the solid oxide fuel cell stack are preheated to a predetermined temperature by heat exchangers provided therein and uniformly distributed over the entire anode and cathode reaction surfaces of unit cells to improve the use efficiency of a fuel cell and in which the sealing of the fuel cell stack is effectively maintained even under high temperature and high pressure conditions to ensure the safety of the fuel cell and increase its durability.
2. Description of the Related Art
In general, a fuel cell is a device that generates electricity by electrochemically reacting fuel (hydrogen) and oxygen in the air and is a high efficiency generator that can continuously generate electricity as long as the fuel is supplied. Unlike conventional generators which require a series of generation processes such as “fuel combustion→steam generation→turbine driven→generator driven”, the fuel cell does not require any driving mechanism for the combustion of fuel or for the generation and thus has advantages that its efficiency is high and that it does not cause environmental problems such as air pollution, vibration, noise, etc.
Among these fuel cells, a solid oxide fuel cell (SOFC), called a third-generation fuel cell, has a structure in which an anode (fuel electrode) is attached to one side of an electrolyte membrane, a cathode (air electrode) is attached to the other side, and a porous current collector is attached to the anode and the cathode to facilitate the electrochemical reaction. Moreover, the fuel cell includes a separator in which fuel and air channels are formed to supply the fuel and air to the anode and the cathode, respectively, and a sealing member which blocks the flow of fuel and air such that the fuel and air do not flow between the anode and the cathode or between the unit cells and bonds the gap between the respective component layers (see Korean Patent Publication No. 2012-0078393).
In the solid oxide fuel cell having the above-described configuration, when oxygen-containing air is supplied to the cathode and fuel gas such as hydrogen is supplied to the anode, a reverse reaction of water electrolysis occurs through the electrolyte membrane disposed between the cathode and the anode to generate electricity and, during the generation of electricity, water and heat are produced and discharged to the outside. At this time, the voltage of the electricity generated in the unit cell of the fuel cell is not high enough to be useful and thus several cells are typically stacked and connected in series in the form of a stack.
However, in the conventional solid oxide fuel cell having the above-described configuration, the reaction of fuel or air occurs more actively in the unit cells adjacent to an inlet side of the fuel or air channel than in the unit cells adjacent to an outlet side, which thus causes a temperature difference between the inlet side and the outlet side of each unit cell over the entire stack. Moreover, in the case of the conventional fuel cell stack, heat exchangers are provided to preheat the fuel and air to an operating temperature of the cell so as to increase the operating efficiency of the fuel cell. These heat exchangers are provided as a separate structure outside the fuel cell stack. On this account, the entire configuration of the system for operating the fuel cell stack is complicated and requires high installation costs. Moreover, during the operation of the fuel cell stack, the fuel and air preheated by the heat exchangers are not uniformly supplied to the reaction surfaces of the unit cells stacked vertically, which causes a local reaction gas shortage, and thus the reactivity of the stacked unit cells is reduced, thus significantly reducing the efficiency and performance of the fuel cell. Furthermore, according to the conventional fuel cell stack, a sealing member made mainly of a glass material is used to maintain airtightness such that the fuel and air flowing through the channels of the separators, disposed on both sides of the unit cell, are not mixed with each other, and an insulator with airtightness and insulation properties is provided in the other portion of the separator which is not in contact with the unit cell to maintain the sealing of the fuel cell stack. However, when the conventional fuel cell stack using the sealing member made of glass is operated at high temperature and high pressure, a viscous flow of glass occurs, which causes a pressure change in the horizontal direction between the contact surfaces of the unit cell and the stack, thus causing destruction of the unit cells due to the pressure change.